We report on experiences from a case study applying a constraint-based process-modelling and -simulation tool, dcrgraphs.net, to the modelling and rehearsal of railway emergency response plans with domain experts. The case study confirmed the approach as a viable means for domain experts to analyse and rehearse emergency response plans, through the activities of formally modelling the plan and subsequently rehearsing it by simulating that model collaboratively. In particular, the constraint-based modelling notation resulted in a flexible model giving rehearsal participants freedom to explore different ways to proceed, including ways not necessarily anticipated in the paper-based emergency response plans. The case study was undertaken as part of a short research, ProSec, project funded by the Danish Defence Agency, with the aim of applying and developing methods for collaborative mapping of emergency and security processes in the danish public transport sector and their dependency on ICT.